1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern on an impinging light beam.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one pass, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
Flat panel displays can be comprised of two plates that are manufactured separately by a lithographic process and then coupled together to form the flat panel display. For the device to function correctly, features on the two plates are precisely aligned. Even a small mismatch of the two plates can lead to light leakage or other distortion in the optical performance of the display. Such errors reduce the overall yield of the fabrication process, increasing the cost of each unit produced. In order to reduce the likelihood of light leakage, the features formed on one plate are conventionally designed to cover a larger area than necessary, providing for a margin of error. However, this typically results in aperture loss and more power being required to provide the same display intensity.
Therefore, what is needed is a manufacturing method in which the yield of a fabrication process for a device comprised of two substrates is improved without sacrificing the performance of the device produced thereby.